This invention relates to a reproduction apparatus for a magneto-optical recording medium.
Recently a magneto-optical recording system has been developed which utilizes a magneto-optical effect, such as a Kerr effect and Faraday effect. In this recording system a recording is performed on a recording medium of magnetic material in a direction perpendicular to its plane through a magnetization in a sense corresponding to recording information. On reproduction a laser light beam (a linearly polarized light beam) is directed to a disk where it is reflected. Since the oscillation plane of the reflected light beam is rotated, in a mutually opposite direction, by a corresponding angle in accordance with that magnetization direction, the recording information can be read out by detecting the rotation angle of the oscillation plane of the reflection light.
In this case, the recording operation can be performed relatively easily, but the reproducing operation cannot. Since the rotation angle of the oscillation plane is very small, i.e., of the order of .+-.0.3.degree. (a difference of 0.6.degree.), it is difficult to detect the rotation angle with high accuracy. For example, the S/N ratio is lowered by noise components resulting from the non-uniformity of a disk substrate and of a magnetic film of the disk. It would, therefore, be difficult to perform a reproduction without involving any erroneous recording information.
"Technical Research Report" vol. 83 No. 197, 1983 of "The Institute of Electronics and Communication Engineers of Japan" discloses "magneto-optical disk system to file memory" which is a conventional reproducing apparatus for a magneto-optical recording medium. FIG. 1 is a block diagram showing a summary of a conventional optical system. Optical pickup 3 is located opposite to one face of magneto-optical disk 2, made of a magnetic material, which is rotated by spindle motor 1. Pickup 3 is disposed on carriage 4 which in turn can be moved by an .alpha. belt, voice coil motor, etc. in a radial direction of disk 2 to permit a tracking or track access.
Laser diode 5 as a monochromatic light source is held within pickup 3. A P- or S-polarization laser beam (linearly polarized wave) from laser diode 5, after being transformed by collimator lens 6 into a parallel beam, is directed to shaping prism 7 where it is transformed into a circular beam. The resultant beam is directed through beam splitter 8 to object lens 9 where it is projected as a spot beam on one face of disk 2. Field coil 10 is located on the side of the other face of disk 2 in such a manner as to be opposed to pickup 3. Upon erasure and recording, a magnetic field is generated in a predetermined direction whereas, upon reproduction, the magnetic field is reversed in its direction by inverting the direction of the current flowing through coil 10. Magnetic coil 10 can be moved in the direction of the diameter of disk 2 as in the case of carriage 4.
Information is recorded with a corresponding magnetization occurring in the direction perpendicular to the face of disk 2. Depending upon whether a magnetization direction is upward or downward, the oscillation plane of the reflective laser beam is rotated by an angle .+-..theta. (a tiny angle) due to the Kerr effect. The reflective beam, after passing through object lens 9, falls on beam splitter 8 where part of the beam is reflected and directed to .lambda./2 plate 11. The .lambda./2 plate 11 is so designed as to permit the oscillation plane of the reflective beam (in the absence of any rotation due to the Kerr effect) to be oriented in an angular direction of 45.degree., i.e., in a position intermediate between P- and S-axes of the polarized beam splitter.
When a laser beam illuminates an information recording section, the intensity vector of that reflective beam which is incident onto beam splitter 12 is
oriented in an angular direction of 45.degree..+-..theta., i.e., in a position intermediate between the S-axis and P-axis as at A.sub.+ and A.sub.- in FIG. 2, depending upon whether or not the illuminated portion of disk 2 is magnetized upwardly or downwardly. If polarized beam splitter 12, serving as an analyzer, is so designed as to permit a P-axis component to be passed and an S-axis component of the reflective beam to be reflected, then the P- and S-axis components of the reflective beam are received by PIN diodes 13a and 13b, respectively.
When disk 2 is rotated by spindle motor 1 and the magnetized direction of the information recording section of disk 2 is reversed in accordance with the recording information, then an amplitude-modulated beam with a difference (Ap in FIG. 2) between P-axis components of light intensity vectors A.sub.+ and A.sub.- as an amplitude is incident onto photodiode 13a and an amplitude-modulated beam with a difference (As in FIG. 2) between S-axis components of light intensity vectors A.sub.+ and A.sub.- as an amplitude is incident onto photodiode 13b. A photoelectric conversion signal a having a pulse amplitude Ap as indicated by a leading half of that pulse train in FIG. 3A is delivered from photodiode 13a and a photoelectric conversion signal b having a pulse amplitude As as indicated by a leading half of that pulse train in FIG. 3B is delivered from photodiode 13b. Since, as set forth above, the oscillation plane of the reflective beam (in the absence of any rotation due to the Kerr effect) is rotated by .lambda./2 plate 11 in the angular direction of 45.degree., i.e., in the position intermediate between the S- and P-axes of polarized beam splitter 12, photoelectric conversion signals a and b have the same pulse amplitudes (Ap, As).
In general, when an amount of laser beam from laser diode 5 varies, noises are contained in the photoelectric conversion signals a and b as shown in FIGS. 3A and 3B. Since, however, the noises contained in the photoelectric conversion signals a and b of the P- and S-axis components are in phase with each other and the pulses contained in the signals a and b have equal amplitudes Ap and As, but opposite phases with respect to each other, then those noises contained in the signals a and b can be eliminated by a differential circuit shown in FIG. 4.
That is, the signals a and b which are output from pickup 3 are input to adder 15 and subtracter 16. Since the pulses contained in the signals a and b are opposite in phase to each other, adder 15 produces a signal c with the amplitudes Ap and As canceled with respect to each other as shown in FIG. 3C. Since the noises contained in the signals a and b are in phase with each other, the output signal c of adder 15 contains a noise having double that amplitude as shown in FIG. 3C. On the other hand, an output signal d of subtracter 16 contains a pulse with the amplitudes Ap and As added together and contains almost no noise as shown in FIG. 3D.
If information is only magneto-optically recorded on magneto-optical disk 2, no problem arises therefrom. In practice, information may sometimes be only optically recorded on optical disk 2. Generally, it is necessary that a track number and sector number be initially recorded on optical disk 2. These address information items are often recorded in the form of uneven information pits (pits are press-worked using a female mold in a stamper) in order to mass-produce disks. The information pit is hereinafter referred to as a pre-pit. Depending upon whether or not there is a pit on the disk, the reflectivity varies. In the conventional magneto-optical disk, ordinary recording information is recorded by a magneto-optical recording method and address information is recorded by an optical recording method. In practice, various information items are often mixed on the magneto-optical disk and recorded by both the magneto-optical method and optical recording method.
Such address information items in the form of pre-pits are comprised of only the intensity-modulated components which vary in the intensity of the reflective beam, depending upon the presence or absence of pits. The oscillation plane of the reflective beam does not vary (rotate). The intensity vector of the beam reflected from a pre-pit section is represented by I.sub.1, I.sub.0 in FIG. 2, due to the presence or absence of the pit. In this case, the magnetization direction of the pre-pit is upward and thus the direction of the vectors I.sub.0 and I.sub.1 is in agreement with that of the vector A.sub.+. When the pre-pit section on the disk is being read out, the output signals of photodiodes 13a and 13b appear one as a pulse with an amplitude, as shown in FIG. 3A, corresponding to an amplitude difference (Ip in FIG. 2) between the light intensity vector components I.sub.1 and I.sub.0 on the P-axis, and one as a pulse with an amplitude, as shown in FIG. 3B, corresponding to an amplitude difference (Is in FIG. 2) between the light intensity vector components I.sub.1 and I.sub.0 on the S-axis.
The reproduction signals a and b are in phase with each other upon reading the pre-pits on the disk. Since Ip&gt;Is, the pulse contained in the reproduction signal from the corresponding pre-pit section on the disk is output from adder 15 as a pulse c of a greater amplitude Ip+Is, as shown in FIG. 3C and from subtracter 16 as a pulse d of a smaller amplitude Ip-Is, as shown in FIG. 3D.
The pulse d is free from noises and much smaller in amplitude when the pre-pit section is read out than when the ordinary information recording section is read out. It is necessary that the information read out of disk 2 be sent, irrespective of being address information or recording information, to a conventional controller 18 on a common data line. Details of the controller 18 are well known, and do not form a part of the present invention. The use of a separate signal processing system results in a bulkier circuit arrangement. For this reason, the signal d cannot be directly supplied to a bi-level quantization circuit. In the circuit arrangement shown in FIG. 4 adder 17 adds together the signals c and d to produce a signal e as shown in FIG. 3E, and the signal e is supplied to one input terminal of comparator 19 through a signal processor circuit 100 comprised of a low-pass filter, waveform equalizing circuit, automatic gain controller etc., while a reference signal from reference signal generator 21 is supplied to the other input terminal of comparator 19, to produce a bi-level signal for delivery to controller 18.
Since, however, the signal c of adder 15 contains doubly amplified noises, the binary output so obtained involves a degeneration of the S/N ratio which is gained from the output signal d of subtracter 16.
The aforementioned problem can be solved by adding together the output signals of adder 15 and subtracter 16 after having been converted to a bi-level signal. Two signal processor circuits are required for, for example, a low-pass filter, waveform equalizing circuit and automatic gain control circuit, thus producing a cost problem. Furthermore, an error is produced due to noises contained in data from the pre-pit section when the data from the data recording section is converted to a bi-level signal. Conversely, an error is produced due to noises contained in data from the data recording section when the data from the pre-pit section is converted to a binary representation.